The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmWave) frequency spectrum between 3G and 300G Hz for the next generation broadband cellular communication networks. The available spectrum of mmWave band is two hundred times greater than the conventional cellular system. The mmWave wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmWave spectrum has wavelengths ranging from 1 mm to 100 mm. The very small wavelengths of the mmWave spectrum enable large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions.
With recent advances in mmWave semiconductor circuitry, mmWave wireless system has become a promising solution for real implementation. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the mmWave network. In mmWave or high frequency systems, directional antenna is utilized to provide higher gain to compensate the pathloss. Directional antenna can be implemented by phased array with many antenna elements. Beamforming and spatial multiplexing methods can be applied in multiple antenna systems. Analog, digital, or hybrid beamforming technique is used in phased array antenna systems. Channel state information is needed when beamforming or spatial multiplexing is applied.
Channel state information can be obtained by estimating either uplink or downlink pilot training symbols. In beamforming technique, angle of arrival (AoA) is one of the channel state information. By adjusting the values of phase shifters, the beam direction in phased array systems can be steered accordingly. In analog beamforming, a set of phase shifter values can be only applied in one training period. One specific antenna pattern is associated with a set of phase shifter values. N training periods are needed if N antenna patterns (directions) are to be detected. This is time consuming. On the other hand, in digital beamforming, different phase shifter values can be applied by digital signal processing in one training period. Multiple RF chain is needed (NA antennas need NA RF chains), which results in high complexity.
A solution is sought to solve the problem of high data rate processing and high power consumption in digital beamforming as well as the problem of large overhead of switching time for switching beams in analog or hybrid beamforming.